what is the meaning of the --max-flow-ctl

Hi Fan - this setting in pt-archiver ensures that the archiving routine doesn’t overwhelm a PXC cluster by setting a limit to how far replication can lag other members of the cluster.

From the documentation:
https://www.percona.com/doc/percona-toolkit/LATEST/pt-archiver.html#cmdoption-pt-archiver-max-flow-ctl

And background on PXC Flow Control:
https://www.percona.com/blog/2013/05/02/galera-flow-control-in-percona-xtradb-cluster-for-mysql/
What is flow control?

I hope this helps!